With the advent of digital video products and services, such as Digital Satellite Service (DSS) and storage and retrieval of video streams on the Internet and, in particular, the World Wide Web, digital video signals are becoming ever present and drawing more attention in the marketplace. Because of limitations in digital signal storage capacity and in network and broadcast bandwidth limitations, compression of digital video signals has become paramount to digital video storage and transmission. As a result, many standards for compression and encoding of digital video signals have been promulgated. For example, the International Telecommunication Union (ITU) has promulgated the H.261 and H.263 standards for digital video encoding. Additionally, the International Standards Organization (ISO) has promulgated the Motion Picture Experts Group (MPEG), MPEG-1, and MPEG-2 standards for digital video encoding.
These standards specify with particularity the form of encoded digital video signals and how such signals are to be decoded for presentation to a viewer. However, significant discretion is left as to how the digital video signals are to be transformed from a native, uncompressed format to the specified encoded format. As a result, many different digital video signal encoders currently exist and many approaches are used to encode digital video signals with varying degrees of compression achieved.
The primary objective of any digital video signal encoder is to achieve a high degree of compression without a significant loss of video signal. Video signal compression is generally achieved by representing identical or similar portions of an image as infrequently as possible to avoid redundancy. As a result, an image which has only very coarse detail and very few distinct colors can be compressed to a much smaller representation in comparison to a compressed representation of an image with significant amounts of very fine detail and many distinct colors. Unfortunately, video cameras and other video signal acquisition equipment introduce noise into the video signal and, from the perspective of video signal processing, the noise is generally indistinguishable from fine detail in the subject of the video signal. For example, ordinary noise in a monochromatic image may be indistinguishable from the fine detail and texture of a terrycloth towel photographed up close.
In addition, noise can have a more severely adverse effect on the degree to which a motion video signal can be compressed than the effect on such compression of fine detail in the subject matter of the motion video signal. In particular, a video image with fine detail frequently includes picture elements, i.e., pixels, which differ only incrementally from one another and which are identical to many other pixels representing other portions of the fine detail. For example, a video image of the fine detail in a knitted sweater typically includes a number of pixels which represent different shades of the same color, or of a few colors, and each shade is represented by a substantial number of pixels. During compression, e.g., using a discrete cosine transformation (DCT), such an image can be compressed quite significantly since different shades of the same color or a few colors are typically grouped into a narrow spectrum and the remainder of the spectrum is discarded. Noise has a particularly adverse effect on such compression since the noise is frequently unrelated to the subject matter of the video image and frequently results in pixel values being transformed outside the narrow spectrum such that greater portions of the transformed pixel data spectrum must be included in the compressed video signal.
To achieve both enhanced image quality and greater compression, video signal encoders frequently filter a video signal prior to encoding the video signal. However, the use of a particularly strong filter achieves greater compression at the expense of greater signal loss, and a particularly light filter preserves more of the original signal at the expense of a smaller degree of compression. One such system is described in U.S. Pat. No. 5,231,484 to Gonzales et al. for "Motion Video Compression System with Adaptive Bit Allocation and Quantization" dated Jul. 27, 1993 (the '484 patent). The '484 patent describes a system which uses an estimation of error in a macroblock of an MPEG encoded frame to determine how strong a filter to apply to the macroblock. However, the estimation of error in the '484 patent does not adequately distinguish noise in the image of the frame from detail in the image. In addition, the filtering performed by the system described in the '484 patent is a smoothing, spatial filter which suffers from unacceptable levels of signal loss in particularly noisy images.
What is needed is a digital video signal encoder which can better distinguish between noise and fine detail in the video signal and filter the video signal in such a way that maximizes elimination of noise to thereby simultaneously improve the quality of the video signal and maximize compression of the video signal.